1. Field of the Invention
The present invention relates to a light emitting device using a light emitting element having a layer that contains an organic compound between a pair of electrodes (hereinafter, referred to as a layer containing an organic compound), on which an electric field is applied to obtain fluorescence or phosphorescence, and to a method for manufacturing such a novel light emitting device. Here, the term “light emitting device” used in the present specification represents one of image display devices, light emitting devices, and optical sources (including lighting units). In addition, the light emitting device may be any kind of the light emitting devices, such as a module on which a flexible printed circuit (FPC), a tape automate bonding (TAB), or a tape carrier package (TCP) is attached; a module where a print-wiring plate is mounted on the tip of a TAB tape or TCP; and a module on which an integrated circuit (IC) is directly mounted on a light emitting element by a Chip-On-Glass (COG) system.
2. Description of the Prelated Art
In recent years, the attention has been focused on technology for constructing a thin film transistor (TFT) using a semiconductor thin film (with a thickness in the range of about several nanometers to several hundred nanometers) formed on a substrate having an insulating surface. The thin film transistors have been widely applied in electronic devices such as ICs and electric optical devices. In particular, the development of such a transistor has been hurried for providing a switching element of an image displaying device.
A light emitting element using an organic compound as a luminous body, characterized by its thin thickness, light weight, fast response, low-voltage DC drive, and so on, has been expected to its application in a next-generation flat panel display. Furthermore, a display device in which light emitting elements are arranged in a matrix configuration has been considered more advantageous than the conventional liquid crystal display device because of being superior in visibility with a wide angle of visual field.
The light emitting element having an organic compound comprises a layer that contains the organic compound from which electro luminescence can be generated by the application of an electric field (hereinafter, it is referred to as an EL layer), and an anode and a cathode. A light emitting mechanism of the light emitting element may be as follows. That is, when a voltage is applied on the organic compound layer being sandwiched between a pair of the electrodes, an electron injected from the cathode and an electron hole injected from the anode are brought into recombination with each other at the luminescence center of the organic compound layer to form a molecular excitation. Subsequently, the light emission is occurred by discharging energy when the molecular excitation returns to a ground state. There are two types of the excitation state known in the art, an excited singlet state and an excited triplet state. The light emission may be allowed in either state.
The light emitting device constructed of a plurality of light emitting elements being arranged in a matrix configuration may be operated by one of well-known driving systems such as a passive matrix drive (a simple matrix type) and an active matrix drive (an active matrix type). However, when the pixel density increases, it may be preferable to use the active matrix type system in which a switch is provided for every pixel (or every dot) because it can be driven at a low voltage.
The active matrix type light emitting device maybe designed in two different ways depending on the direction of light radiation. One of them is that light emitted from an EL element passes through an opposite substrate and radiates into the eyes of the observer (i.e., a lower-side radiation type). In this case, the observer is allowed to recognize an image from the opposite substrate. The other is that light emitted from the EL element passes through the substrate of this element and radiates into the eyes of the observer (i.e., an upper-side radiation type). In this case, the observer is allowed to recognize an image from the direction of the EL element.
Furthermore, monomeric and polymeric materials have been studied for an organic compound to be provided as an organic compound layer (i.e., a light emitting layer in the strict sense), which may be the center of the light emitting element. Among them, the attention has been focused on the polymeric material because of its high thermostability and convenience in handling compared with the monomeric material.
For forming a film from an organic compound, an evaporation method, a spin-coating method, and an ink-jet method have been known in the art. Among them, for realizing a full-color image formation using a polymer material, the spin-coating method and the ink-jet coating have been particularly known in the art.
The light emitting element having an organic compound has a disadvantage in that it tends to be deteriorated by various factors, so that the greatest problem thereof is to increase its reliability (make longer its life span).